Which PCB Connectors Are Best?

There are many approaches to flexible (i.e., versatile) connections, but it appears that European offerings are slowly taking over the market.

At the outset of this blog, let me make full disclosure. I used to work for one of the companies that I mention in this article. Also, my current employer was spun off from, and still has a business arrangement with, that company. This arrangement has kept me occupied for 22 years. As a result, I have greater access to Weidmuller versions of the terminals I discuss than to others, so this blog is likely to appear biased in that regard. Although I will try and stay balanced, it must be said that the contents of this (and all my other blogs) are my opinions alone, and not those of my employers -- current or previous.

It seems to me that the connection requirements for industrial electronics are different than the other market sectors, with the possible exception of hobbyist electronics. In the industrial market, integrators take a bunch of functional modules and combine them with Programmable Logic Controllers (PLCs) and sensors to make a complete system whose various modules are connected by long lengths of cables. Often, some portions of the system are installed long before the others and they have to be hooked up on site. Changes in specifications and plant upgrades also mean that there must be some flexibility in the interconnections.

Industrial products do not approach the high volumes of the automotive and consumer market places and are much less cost-sensitive, trading this for interface flexibility. Mostly, the whole industrial package is concentrated and housed in cabinets. They are typically mounted on rails, and the European TS35 and TS32 rails seem to me (sitting on the fringes of the market) to be the most dominant. There are many variations of terminal blocks that mount on these rails (I may write another blog on those in the future).

There are many approaches to flexible (by which I mean versatile) connections, but it seems to me the European offerings appear to be slowly taking over the market, despite established approaches like barrier strips in the North American industry. There are also other common approaches, such as the 0.1" or 0.156" headers made by Molex, Amp, and many others which require crimp tools. Then there are spade terminations and their ilk, and you may even remember the low-end Fahnestock clips.

The first issue in making field connections is the service personnel typically have only three tools, a BFS, a BFH, and a BFW (a screwdriver, a hammer and a wrench -- the "B" stands for "big" and I leave the "F" to your imagination). The bigger the connectors used, the happier the installers are. The second issue is terminating the wire, which may also require specialized crimping tools. This is part of what the nascent industry (in the 1950s and 60s) was addressing, but -- as we shall see -- things have come full circle.

One of the problems is getting high density connections off a board and connected to the field wiring. An approach was to bring out connections from the PCB via a standard high density cable, and to provide interposing external terminals to connect to as you can see in Figure 1.

Figure 1. A 37-way D-subminiature to screw terminal adapter. The connectors are double stacked to improve the density, but are not pluggable.(Click here to see a larger image.)

Variations also allow for industry standard flat IDC (insulation displacement connector) cable to screw terminal. This kind of cable can be inflexible; it only comes in fixed lengths, and it needs specialized crimping/soldering equipment. Since it mounts horizontally on the rail, it also uses up rail space, which is often an issue as a result of under-design. The biggest issue is the isolation and current capabilities of the cables.

From my biased perch, the lion's share of the PCB screw terminal market appears to be owned by two German companies, which are, in no particular order, Weidmuller (Weidmueller in the USA) and Phoenix Contact (if you disagree, please feel free to contradict me in the comments below). There are a few more German companies (Wago and Weco, for example) and some US and Asian (try Degson Electronics) manufacturers as well.

It used to be easy to tell who made a terminal by its color. Weidmuller's offerings were orange or black, while and Phoenix's were green. More recently, Phoenix has introduced some in black, and there are now many mimics of the original shades. I should also mention that there are some higher-end housings that include connectors that are integral to the package as you can see in Figure 2.

Figure 2. A housing with specialized pluggable connectors.(Click here to see a larger image.)

There is an ever-expanding range of PCB terminals. They started out with non-pluggable screw terminals (see Figure 1 and Figure 3), and today they can be purchased with different numbers of poles. Furthermore, some can even be joined together to make terminal blocks of any length.

Figure 3. Non-pluggable screw terminals; note the ability to add clip-in tags to identify each pole of the connector.(Click here to see a larger image.)

The use of fixed terminals means that -- when removing and re-installing a module -- you have to individually unscrew every terminal and then remember how to reconnect the wires. As a result, the trend is to pluggable connectors, but there are exceptions, especially when you are using connectors for very high currents like those in Figure 4.

Your choice of which pluggable connector to use is governed by the voltage and current of the signal. Often, connectors are interchangeable between manufacturers, sometimes down to the plug of one manufacturer fitting the socket of another. The highest capacity pluggable connector I know (Figure 5) is rated at 54 amps, but I don't imagine many of you are involved in that ballpark, so I will quickly move on, not the least that it also exceeds my comfort level by many orders of magnitude.

It gets used for everything around here. I have seen it used to reinforce the shelves on a bookcase! ;-)

We actually have another non-German variation as well. It is perforated every ~1/2" so that you can easily cut it to your desired length with a basic hand tool. It sounds like you have the pneumatic guillotine in house, but this is quite handy for the small jobs that we do, like test jigs. Please forgive the plug!

I suspect Phoenix in dominant in Germany, because when we had a German intern, that's all he ever specified. In the US, I'd say their availability is better than the others.

I am not going to be drawn on this because I don't have any numbers, only biased opinions. But what is interesting is that Phoenix, Weidmuller and Wago are all located within about a 100km circle of each other in Germany. I think I was quoted that 70% of the world market in screw terminals was concentrated there- however that must be 15 years ago now.

-- For me, it's Euro-style all the way. And, I've only seen DIN rail used as a mounting rail (we use as much as possible, plus screw holes for the stuff that need it like servo drives and pneumatic manifolds).

-- As I re-design our machines, I'm moving away from DIN rail terminal blocks, replacing them with custom PCBs. BOM cost is about the same, but labor cost is much less, and reliability should be better. I'm currently using Phoenix 4mm ST series spring clamp DIN Rail fuse holders & terminal blocks for power distribution; IMHO, they work great for this.

-- However, there are some cool DIN rail terminal blocks, such as the Phoenix Combi plugs that plug into Combi DIN rail terminal blocks; current is 24A or 32A, and the price is reasonable.

-- On my custom break out PCBs, I try to maximize use of standard molded cables (e.g. DB or HD cables from servo drive to PCB) or easy to make cables (e.g. IDC ribbon cable from PLC to PCB -- that's why I like PLCs with box headers instead of terminal blocks). The fewer cable connections our techs have to make, the better. Typically, I've used fixed terminal blocks, because of cost and that we don't normally have to swap out boards. In the future, I will look at using more pluggable TBs, but the value has to be there.

-- Industrial automation is a wide field. We don't have BFS techs, and the machines rarely need re-wiring in the field (only occasionally to add stuff).

-- The Phoenix PC35 15mm pluggable terminal blocks are rated at up to 125A and 600V, but a 3 pos header/plug pair will run about $40. That's impressive, although Positronics has Comb-D connectors at 100A.

-- I suspect Phoenix in dominant in Germany, because when we had a German intern, that's all he ever specified. In the US, I'd say their availability is better than the others.

--Availability can be a big issue, when you start getting into unusual terminal blocks, such as inverterted, screw flanges, or such. If I cannot get it easily, I won't use it.

-- One big plus for the more common sizes & styles of headers (like the Phoenix Combicon Classic 5.08 series) is the wide variety of plugs you can use: screw, spring clamp, or IDC in a variety of styles. So you can spec your headers up front, and pick your plugs later.

-- The Phoenix PTSM series is really cute, and are reasonably priced (I bought a few for playing with). IIRC, the plugs are IDC or spring clamp. You can open the spring clamp ones with a 1.5mm flat screw driver. I haven't tested how well they work with stranded wires. The headers are also available in through hole.

-- However, one of the custest connectors is the Omron XN2 e-Con connector; it's a little pricey, but it's a 2mm spring clamp connector. The e-Con is a Japanese connector standard for factory automation, developed originally by Tyco Japan. In the US, the Tyco RITS (IDC), 3M Mini-Clamp (IDC), and Omron XN2 (spring clamp) are available. Anyway, I use these connectors a lot for stage limit sensors, however, we had problems using the IDC models with Panasonic PM sensors (approx wire size 29 AWG), but the XN2's work great.

To put an even finer point on this (what a clever pun, sorry!), the spelling Weidmueller is also an acceptable German spelling. You have the option of using the umlaut or the u followed by an e, even in Deutsch.

Sometimes, when the name Müller is amercanized, I've seen it spelled Miiller or Muiler.

Actually, the German pronunciation would be Videmewler. E before I, say I; I before E, say E.

The change of spelling is simply to reflect that many Latin alphabets (e.g. English) do not contain umlauted characters (excuse the ersatz verb). The Germans are very progressive on this aspect of their language.

Let's all try one: Wienerschnitzel. How does that sound? And another: Weißwein. Yeah, well we run into another problem here with the ß character, which the Germans would allow us to replace with ss, i.e. Weisswein.

Aubrey....re chocolate strip. I was introduced to this term in the police in Rhodesia as an apprentice Radio Tech. We used to use lots of it in vehicle radio installations.

I have a few of the pluggable type. They came from emergency exit lights to connect the switched live / unswitched live / neutral / earth. In one case both halves were fixed to the case in such a way that when you pulled the two halves of the case apart it disconnected the wires.

I was a fan of Weidmuller connectors decades ago. Then we discovered there were many competitors of similar connectors. After comparing a half dozen lines we zeroed in on the Phoenix Contact line which had many superior details.

You are correct!! No point in having gold plated terminal if the user still inserts tinned copper wire or tinned-ferruled wires...then what makes more sense? Not to have gold plating as that would add unnecessary cost burden, if the users are allowed to insert the wires with ferrule materials of their choice?

I have a friend who calls these chocolate block connectors. He too is orignially from England (and then Kenya and South Africa) and is the only other time that I have heard the nomenclature. Is anyone else familiar with the term?

I have actually seen versions of these with a pluggable connector in the middle. I wonder if they are still made?